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A thorn of a tangerine tree (1882)

It is Christmas 1882 in the Sicilian town of Messina. A visiting Russian scientist called Ilya (Elie) Mechnikoff is looking at a tangerine tree which has been decorated for his children instead of the usual pine Christmas trees of northern Europe. Metchnikoff has an idea and plucks a few small thorns from the tree for an experiment he has in mind – and so begins a story that leads him to one of the greatest discoveries in immunology.

Mechnikoff had fled to Messina after resigning his appointment at the local university in Odessa during a period of reactionary government policies in Russia which followed the assassination of Tsar Alexander II. He went to work at a private laboratory in Sicily to continue his research on comparative embryology, using the larvae of starfish as one of his experimental subjects. In the process he discovered the phenomenon of “phagocytosis” – the extraordinary and vital ability of some immune cells to gobble up invading microbes.

The thorns of the tangerine tree come into the story because Mechnikoff had observed that some of the cells of the starfish larvae were mobile and he thought they may be part of a cellular defence system. So he pierced the larvae with the tangerine thorns and the next morning found the tips were surrounded by these mobile cells. He already knew that inflammation in animals with a blood system causes white blood cells to escape from the blood vessels and he thought that these leucocytes might take up and digest bacteria that invade the body. Here he came across visual evidence to support his theory.

On his way back to Odessa in 1883, he dropped in to see his colleague Carl Friedrich Wilhelm Claus, professor of zoology at the University of Vienna, who suggested the term “phagocyte” or “eating cell” for a mobile cell that surrounds and swallows up material in this way. Mechnikoff later observed phagocytes in other organisms, such as the small fresh-water crustacean Daphnia, in which he found them attacking fungal spores. Mechnikoff is also credited with discovering macrophages, which means “big eaters”, a type of white blood cells that engulfs and digests cellular debris, foreign substances, microbes, cancer cells and indeed anything else that does not have the right type of membrane proteins indicating a healthy body cell.

Mechnikoff established the concept of “cell-mediated immunity” and won a share of the 1908 Nobel prize in medicine or physiology for his work on the immune system. His scientific success also personally gave him a new lease of life after he twice tred to commit suicide after a series of personal disasters, notably the death of his first wife and serious illness of his second wife. 

It was the thorn of a tangerine tree that helped him to continue working productively until his death in 1916.